routerrpc: extend HTLC forward interceptor resp with modification fields

This commit extends the forward HTLC intercept response with fields that
can be used in conjunction with a `ResumeModified` action to modify the
intercepted HTLC p2p message.

lnrpc/routerrpc/router.swagger.json

@@ -1474,9 +1474,27 @@
         "failure_code": {
           "$ref": "#/definitions/FailureFailureCode",
           "description": "Return the specified failure code in case the resolve action is Fail. The\nmessage data fields are populated automatically.\n\nIf a non-zero failure_code is specified, failure_message must not be set.\n\nFor backwards-compatibility reasons, TEMPORARY_CHANNEL_FAILURE is the\ndefault value for this field."
+        },
+        "in_amount_msat": {
+          "type": "string",
+          "format": "uint64",
+          "description": "The amount that was set on the p2p wire message of the incoming HTLC.\nThis field is ignored if the action is not RESUME_MODIFIED or the amount\nis zero."
+        },
+        "out_amount_msat": {
+          "type": "string",
+          "format": "uint64",
+          "description": "The amount to set on the p2p wire message of the resumed HTLC. This field\nis ignored if the action is not RESUME_MODIFIED or the amount is zero."
+        },
+        "out_wire_custom_records": {
+          "type": "object",
+          "additionalProperties": {
+            "type": "string",
+            "format": "byte"
+          },
+          "description": "Any custom records that should be set on the p2p wire message message of\nthe resumed HTLC. This field is ignored if the action is not\nRESUME_MODIFIED."
         }
       },
-      "description": "*\nForwardHtlcInterceptResponse enables the caller to resolve a previously hold\nforward. The caller can choose either to:\n- `Resume`: Execute the default behavior (usually forward).\n- `Reject`: Fail the htlc backwards.\n- `Settle`: Settle this htlc with a given preimage."
+      "description": "*\nForwardHtlcInterceptResponse enables the caller to resolve a previously hold\nforward. The caller can choose either to:\n- `Resume`: Execute the default behavior (usually forward).\n- `ResumeModified`: Execute the default behavior (usually forward) with HTLC\nfield modifications.\n- `Reject`: Fail the htlc backwards.\n- `Settle`: Settle this htlc with a given preimage."
     },
     "routerrpcGetMissionControlConfigResponse": {
       "type": "object",
@@ -1763,9 +1781,11 @@
       "enum": [
         "SETTLE",
         "FAIL",
-        "RESUME"
+        "RESUME",
+        "RESUME_MODIFIED"
       ],
-      "default": "SETTLE"
+      "default": "SETTLE",
+      "description": " - SETTLE: SETTLE is an action that is used to settle an HTLC instead of forwarding\nit.\n - FAIL: FAIL is an action that is used to fail an HTLC backwards.\n - RESUME: RESUME is an action that is used to resume a forward HTLC.\n - RESUME_MODIFIED: RESUME_MODIFIED is an action that is used to resume a hold forward HTLC\nwith modifications specified during interception."
     },
     "routerrpcRouteFeeRequest": {
       "type": "object",